how can I prevent that TabPanel fires the "tabchange" event on rendering

how can I prevent that TabPanel fires the "tabchange" event on rendering?

I have a Grid in one of my tabs, on window resize or tabchange i set the height/width of the grid. That works, but in first time (afterrender) the grid is not visible. So i have to resize the window in height to see the grid...

in a previous version of my programm i used the "Ext layout System" but for some reason the just resize the height, not the width...
Now, the Window is the only one that have a layout property (fit), the grid have the property "forceLayout: true"